1. Field of the Invention
The present invention is related to the field of electric wireline formation testing tools. More specifically, the present invention is related to systems for recovering samples of fluid from earth formations which are substantially maintained in their original phase composition.
2. Description of the Related Art
Electric wireline formation testers are known in the art for withdrawing samples of fluids from pore spaces of earth formations penetrated by wellbores. The formation testing tools known in the art typically include a sample tank into which the sample of fluid withdrawn from the earth formation can be discharged and then transported back to the earth's surface for laboratory analysis.
Some of the earth formations from which fluid samples are withdrawn can be located at significant depths within the earth. As is understood by those skilled in the art, the temperature and the pressure of the fluids within the pore space of a particular earth formation can be related to the depth of the particular formation within the earth. As is also understood by those skilled in the art, native fluids within the earth formation can include hydrocarbons. The chemical composition of the hydrocarbons within any particular formation fluid is typically unique to the particular formation and is related to the temperature and pressure to which the formation was subjected during the geologic processes which generate and accumulate the hydrocarbon in the particular earth formation.
It is known in the art for hydrocarbons in earth formations to undergo phase changes when pressures and temperatures on the hydrocarbons are reduced. Phase changes can include condensation of gaseous hydrocarbon into liquid and precipitation of solid hydrocarbon which is in solution in liquid hydrocarbon. The pressure and temperature at which a particular phase change occurs depend on the concentration of liquid and gas in solution. Phase changes which can occur while acquiring a fluid sample for laboratory analysis can so alter these concentrations that the laboratory analysis of phase behavior is subject to error.
Phase changes can also reduce the efficiency of production by reducing the effective permeability of the earth formation with respect to the flow of hydrocarbon. For example, liquid resulting from condensation has higher viscosity than gas. For any value of differential pressure and formation permeability, higher viscosity results in lower flow rates.
Production of hydrocarbon from the formation at excessive rates can cause such phase changes particularly because of the drop in temperature associated with high rates of production.
It is useful to the wellbore operator to be able to determine the composition of the hydrocarbons in the formation as closely as possible. It is particularly useful to the wellbore operator to be able to determine temperatures and pressures at which phase changes in a particular hydrocarbon sample may occur. Determining the hydrocarbon composition and the conditions under which phase changes occur can enable the wellbore operator to design production equipment for the wellbore so that the efficiency with which the hydrocarbons are extracted from the formation is optimized, as is understood by those skilled in the art.
It is known in the art to withdraw samples of fluid from the earth formation with a wireline formation testing tool having a so-called variable pressure control ("VPC"). VPC is described for example in U.S. Pat. No. 4,507,957 issued to Michaels et al. The VPC in the Michaels et al '957 patent enables the tool operator to cause the fluid to flow from the formation into a sample tank at a sufficiently slow rate so that the fluid pressure is typically maintained above condensation or precipitation pressures.
U.S. Pat. No. 5,303,775 issued to Michaels et al describes a method for pumping fluid from the formation into the sample tank at pressures above the native fluid pressure in the earth formation so that some compensation for cooling of the fluid sample can be obtained. Cooling results when the testing tool is withdrawn from the wellbore to the earth's surface. Sometimes the cooling can be sufficient to cause a phase change in certain fluid samples. Compensation by overpressurizing the sample can reduce or eliminate temperature induced phase change in the fluid sample.
A drawback to the overpressurizing method for reducing phase change in hydrocarbon fluid samples is that some samples have compositions which will still undergo phase change as a result of cooling despite overpressurizing the sample. Phase change in the sample may preclude the wellbore operator from determining the composition of the hydrocarbon as it exists in its native state in the earth formation, making it difficult to design appropriate production equipment.
Accordingly, there is a need for an electric wireline formation testing tool which can maintain the temperature of a fluid sample in its test tank as near as possible to the native temperature to reduce the possibility of phase change in the fluid sample.